As is known in the art, a frequency selective limiter (FSL) is a nonlinear passive device that attenuates signals above a predetermined threshold power level while passing signals below the threshold power level. One feature of the FSL is the frequency selective nature of the high-power limiting: low power signals close in frequency to the limited signals are substantially unaffected (i.e., the FSL does not substantially attenuate such signals).
A typical implementation of an FSL includes a stripline transmission line structure using two layers of dielectric material disposed about the stripline, with the stripline having a fixed length and a fixed width along the length of the FSL. Such structures are relatively simple to fabricate and provide adequate magnetic fields to realize a critical power level of approximately 0 dBm when using a single crystal material. One method of reducing the threshold power level is to use a lower-impedance stripline at the cost of degraded return loss. An external matching structure can be used to improve the impedance match, but this technique reduces the bandwidth and increases the insertion loss of the FSL. Permanent biasing magnets can be mounted to, or near, the FSL structure to produce a bias field. The strength of the magnetic field within the structure establishes the operating bandwidth of the limiter.